Radioiodinated glucose analogues for use as imaging agents

ABSTRACT

A radioiodinated branched carbohydrate for tissue imaging. Iodine-123 is stabilized in the compound by attaching it to a vinyl functional group that is on the carbohydrate. The compound exhibits good uptake and retention and is promising in the development of radiopharmaceuticals for brain, heart and tumor imaging.

This invention relates to a radiopharmaceutical in the form of aradioiodinated branched carbohydrate that can be used for brain, heart,and tumor imaging. This invention was prepared pursuant to a contractwith the United States Department of Energy.

BACKGROUND OF THE INVENTION

Imaging of organs is a technique that utilizes a compound that is activein the body and which has been tagged with a radioactive element whichcan be detected using a special camera, the type camera being dependentupon the type of emissions from the radioactive element. This diagnostictool is useful in studying reactions that take place in various bodyorgans and can provide early detection of unusual reactions that can beindicative of disease. The chemical compound that carries theradioactive tag or label must travel through the organ that is beingstudied and must undergo a reaction that will provide informationrelated to the state of health of the organ or the presence of a tumor.Glucose is the primary energy source of the brain and is also animportant metabolic substrate for a normal heart and is therefore a goodchoice when selecting a vehicle to be tagged.

In the search for a suitable radiopharmaceutical for imaging purposesfluorine-18 has been used as a tag to form the compoundfluoro-2-deoxy-D-glucose. Although the tagged compound is easilysynthesized, fluorine-18 is available only from a cyclotron or reactorand it has a half-life of only 110 minutes; therefore, it is not readilyavailable and its emissions are of limited duration. In addition tothese problems, it also emits positrons having 511 keV gamma rays whichcannot be detected using commonly available cameras. Considering theirshort half-lives, these compounds cannot be stored so they must be made,used, and measured at a single location, and since most hospitals do nothave cyclotrons, reactors or special cameras for detecting thefluorine-18 compounds, this compound is not a practical choice fordiagnostic purposes on a large scale. Another radioactive element,carbon-11, also would be easily incorporated into a glucose molecule;however, its half-life is only 20 minutes and it also emits 511 keVpositrons.

A better choice is iodine-123 which emits particles having 159 keV thatcan be detected by all nuclear medicine imaging cameras, and since italso has a half-life of 13.3 hours, it can be stored for a longer periodof time than can radioactive fluorine or carbon. However, use of iodinepresents a problem because, due to its chemical properties, iodine isnot easily attached to glucose or a glucose-like molecule. Therefore,there is a need to develop a process to synthesize iodine-123 taggedcompounds which undergo reaction in desired organs or tumors in thebody.

SUMMARY OF THE INVENTION

In view of the above need, it is an object of this invention to providebrain, heart, and tumor imaging agents that have long lives and are easyto detect using commonly available nuclear medicine imaging cameras.

It is another object of this invention to provide radioiodinated glucoseanalogues and a method for making them.

It is a further object of this invention to stabilize iodine on acarbohydrate glucose analogue so the compound does not becomedeiodinated in the body. Other objects and advantages of this inventionwill become obvious to a person skilled in the art upon study of thespecifications and the appended claims.

The invention is an imaging agent comprising an administering medium anda compound which is a glucose anlogue to which is covalently attached avinyl group to which is covalently attached the radioisotope I-123.

The invention is also a process for radioimaging using a suitablesolution containing a radioiodinated glucose analogue.

The compound that was prepared to test stability and activity showedfavorable results when tested on the tissues of female rats and it isbelieved that this process and this compound constitute significantfirst steps in further development of effective imaging agents that canbe easily manufactured and stored for a period of time and used inconjunction with conventional nuclear medicine imaging photography.

BRIEF DESCRIPTION OF THE DRAWING

A schematic drawing detailing the procedural steps and chemicalintermediates of the process of making the compound described in theexample of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In organ imaging, an element that emits radiation is tagged to acompound that travels through the body and enters the organ or area ofthe body which is the subject of investigation. For study of the brain,the heart, and tumors, glucose is a suitable carrier for the radioactiveelement since it feeds the brain as well as tumors and is also animportant metabolic substrate for a normal heart. It is required fornormal cerebral and myocardial metabolism; thus, the measurement ofregional difference in uptake and clearance of a glucose-like compoundcarrying a radioactive tag could be an accurate and unique technique ofdetecting subtle differences in regional glucose metabolism that wouldcorrelate to the onset and progression of several forms of brain andheart disease.

Glucose enters the brain by crossing the blood-brain barrier; however,it cannot readily cross the barrier without assistance and thus requiresan enzyme for transport. In theory, if a radioactive tag is placed on acompound that the enzyme will recognize as glucose, the enzyme willtransport the radioactive compound across the blood-brain barrier.Fluorine-18 and carbon-11 can both easily be attached to a glucose-likecompound since fluorine-18 is similar to a hydroxy group in size andelectronegativity and carbon-11 can easily replace a nonradioactivecarbon of the glucose molecule. However, when iodine-123 is chosen as atag difficulties arise. Iodine has unfavorable chemical reactivityproperties and is not easily attached to a glucose like compound; italso forms a weak carbon-iodine sp³ bond, whereas both fluorine andcarbon form strong sp3 carbon bonds. This weak bond of the iodineinhibits synthesis of a radiopharmaceutical and also promotes chemicalas well as in vivo deiodination after synthesis.

One approach is to stabilize the iodine by changing the sp³ bond to ansp² bond using a method that would not change the compound to such agreat extent that the enzyme fails to recognize it as glucose. Thisinvention provides a method of stabilizing an iodine by attaching it toa vinyl functional group attached to the carbohydrate while notaffecting the carbohydrate to the extent that the enzyme does notrecognize it and transport it across the blood-brain barrier. In thisprocess, the vinyl group is first attached to the carbohydrate, and theiodine is subsequently attached to the vinyl functional group. In thecourse of preparing these compounds a new method was discovered ofopening an epoxide ring using an unsaturated Grignard reagent in orderto attach the vinyl functional group.

Glucose analogues were chosen as labeled gamma emitters in thisexperiment with the expectation that they would be transported acrossthe blood-brain barrier into the brain and be active in the brain. Itwas hoped that once in the brain, the substituted glucose analogue wouldundergo a reaction which is the initial step of glycolysis. Ordinarily,the subsequent steps of glycolysis would follow; however, when theanalogue containing a substituted iodine is used, subsequent enzymaticsteps are inhibited resulting in prolonged cerebral retention of theradioactive compound. This allows measurement of the rate of the firststep of glycolysis which is indicative of the health of the organ beingstudied. If the brain is being studied, then a decrease in the ratewould indicate that the brain is not functioning properly. In the caseof a tumor, the indication would be that the rate of reaction isexcessive, leading to overactive growth in a particular area. Thesignificance of the specific rates of activity in the glycolysis processwould be apparent to those skilled in the art.

EXAMPLE

A test radiopharmaceutical methyl-2-deoxy-2-(E)-[¹²⁵I]-iodovinyl-2,4,6-0-triacetyl-β-D-altropyranoside 6 was prepared toestablish that a deoxy substituted iodovinyl carbohydrate could bedelivered to the brain in significant levels with prolonged retention.One novel aspect of the invention was the scission of an anhydro sugarwith the Grignard reagent. This technique was chosen because a varietyof 2-deoxy iodovinyl carbohydrates could be prepared depending on theposition of the anhydro group within the pyranose ring. The process ofthe synthesis is shown in the drawing.

Methyl-2,3-anhydro-4,6-0-benzylidene-β-D-allopyranoside 1 was treatedwith ethynyl magnesium chloride to give4,6-0-benzylidene-2-deoxy-2-ethynyl-β-D-altropyranoside 2. Ethynylmagnesium chloride (EMC) was placed in tetrahydrofuran, a solvent, towhich was added the epoxide having a proportion to EMC of 1 to 10. Thismixture was heated for four hours at reflux, subsequently cooled andadded to water. The product was then extracted with ethyl ether, whichwas subsequently removed by evaporation in a vacuum. Final purificationof the product was done by column chromatography to give compound 2.Hydrostannylation of 2 with tributyl tin hydride, n(Bu)₃ -SnH, gave thekey intermediate, methyl 4,6-0-benzylidene-2-deoxy-2-(E)-(n-Bu)₃SnHC═CH-β-D-altropyranoside 3. Iododestannylation of 3 with sodiumiodide and N-chloro-succinimide gave methyl 4,6-0-benzylidene2-deoxy-2-(E)-IHC═CH-β-D-altropyranoside 4. The processes ofhydrostannylation and iododestannylation are described in the Journal ofMedicinal Chemistry, Vol. 28, p.807, 1985. Hydrolysis of 4 with 20percent trifluoroacetic acid, CF₃ COOH, followed by treatment withpyridine-Ac² 0 gave the desired radiopharmaceutical 6. These processesare commonly known to persons skilled in the art.

The distribution of radioactivity in tissues of female rats at 5minutes, 15 minutes, 30 minutes and 60 minutes after intravenousadministration of the iodinated compound is shown in the followingtable. The level of accumulation of radioactivity in the brain afterinjection of this agent was a significant 1.65 percent dose/g at 5minutes but the blood levels were also high resulting in blood ratios of1.6/1 at 5 minutes. The agent exhibited prolonged retention in thebrain, 0.72 percent dose/g at 60 minutes which is 50 percent whencompared with the peak uptake at 5 minutes. The accumulation of activityin the thyroid was low, 33.5 percent at 60 minutes, which demonstratedthe stability of this agent to in vivo deiodination.

    ______________________________________                                        Distribution of Radioactivity in Rat Tissues                                  at Various Times after Intravenous Administration of                          [.sup.125 I]-Methyl-2-Deoxy-2-(E)--Iodovinyl-2,4,6-O--Triacetyl-              β-D-Altropyranoside                                                               5 MIN  15 MIN    30 MIN   1 HT/R                                     ______________________________________                                        Blood      1.0549   0.9335    0.0037 0.6529                                   Min        1.00     0.089     0.74   0.64                                     Max        1.10     0.96      0.89   0.68                                     S D        0.0420   0.0371    0.0748 0.0266                                   Liver      1.3922   1.3676    1.2320 1.0541                                   Min        1.34     1.33      1.12   0.97                                     Max        1.42     1.39      1.42   1.10                                     S D        0.0380   0.0362    0.1620 0.0748                                   Kidney     1.5513   1.5725    1.6954 1.5722                                   Min        1.50     1.49      1.52   1.41                                     Max        1.59     1.63      1.84   1.74                                     S D        0.0351   0.0648    0.0674 0.0454                                   Lung       1.1948   1.0349    0.8325 0.6632                                   Min        1.09     0.97      0.74   0.66                                     Max        1.33     1.11      0.92   0.67                                     S D        0.1140   0.0710    0.0921 0.0051                                   Thyroid    20.6441  18.9149   25.7085                                                                              33.4920                                  Min        20.06    17.53     22.95  27.79                                    Max        21.27    20.53     30.15  43.47                                    S D        0.6611   1.5134    3.8894 8.6674                                   Brain      1.6525   1.1933    0.8908 0.7237                                   Min        1.56     1.10      0.75   0.71                                     Max        1.73     1.32      1.05   0.75                                     S D        0.0916   0.1161    0.1483 0.0232                                   Heart/Blood                                                                              1.0776   1.1490    1.0809 1.0408                                   Min        1.06     1.08      1.06   1.01                                     Max        1.09     1.23      1.10   1.07                                     S D        0.0129   0.0739    0.0178 0.0292                                   Brain/Blood                                                                              1.5666   1.2775    1.1035 1.1087                                   Min        1.49     1.20      1.01   1.10                                     Max        1.64     1.39      1.18   1.12                                     S D        0.0594   0.0990    0.0838 0.0107                                   ______________________________________                                    

In human applications, the radiopharmaeutical is combined with a salinesolution or another suitable administering medium and is given to thesubject by intravenous injection. Only a trace amount of theradiopharmaceutical is needed, the preferred amount being dependent onthe extent of specific activity desired. Normally, one would want anamount sufficient to provide a distinct image, the exact amountdepending on a number of factors that can be readily determined bypersons skilled in the art.

The radioiodinated iodovinyl substituted carbohydrates offer advantagesover fluorine-18 and carbon-11 labeled glucose analogues that have beenpreviously used for measuring regional glucose metabolism in the brainand heart, since radioiodine is a single photon emitter with a 13.3 hourhalf-life making it detectable using widely available single photonemission computerized tomographic devices. It is also easily preparedfollowing the process of this invention. Other carbohydrates that haveadvantages similar to glucose could also be synthesized. One suchcarbohydrate is mannose which could provide radiopharmaceuticals thatexhibit activity in specific body tissues.

We claim:
 1. An imaging agent comprising: an administering mediumsuitable for intravenous injection and a compound comprising a glucoseanalogue to which is covalently attached a vinyl functional group towhich is covalently attached the radioisotipe I-123, said compound beingpresent in an amount sufficient to produce a radioimage of tissue usingradioimaging techniques.
 2. The imaging agent of claim 1 wherein saidvinyl functional group is attached at position-two carbon of saidglucose analogue.
 3. The imaging agent of claim 2 wherein said glucoseanalogue is the structure ##STR1##
 4. The imaging agent of claim 2wherein said glucose analogue is structure ##STR2##
 5. A process forradioimaging of human tisslue comprising:administering the compositionof claim 1 by intravenous injection of humans; allowing a sufficientamount of time for said composition to enter tissue to be radioimagedand for a radioactive portion of said composition to become immobilizedwithin said tissue; and recording a radioimage of said tissue usingradioimaging techniques.
 6. A process for radioimaging of human tissuecomprising:administering the composition of claim 2 by interavenousinjection of humans; allowing a sufficient amount of time for saidcomposition to enter tissue to be radioimaged and for a radioactiveportion of a said composition to become immobilized within said tissue;and recording a radioimage of said tissue using radioimaging techiques.7. A process for radioimaging of human tissue comprising:administeringthe composition of claim 3 by interavenous injection of a human;allowing a sufficient amount of time for said composition to entertissue to be radioimaged and for a radioctive portion of saidcomposition to become immobilized within said tissue; and recording aradioimage of said tissue using radioimaging techniques.